Membrane-electrode assembly for a fuel cell and a fuel cell system including the same

ABSTRACT

A membrane-electrode assembly for a fuel cell includes an anode and a cathode facing each other and a polymer electrolyte membrane interposed therebetween. At least one of the anode and the cathode includes a conductive electrode substrate and a catalyst layer formed thereon, and the catalyst layer includes a first catalyst layer including a first metal catalyst that grows from the polymer electrolyte membrane toward the electrode substrate and a second catalyst layer including a second metal catalyst covering the first catalyst layer.

CLAIM OF PRIORITY

This application makes reference to, incorporates the same herein, andclaims all benefits accruing under 35 U.S.C.§119 from an application forMEMBRANE-ELECTRODE ASSEMBLY FOR FUEL CELL, METHOD OF PREPARING SAME ANDFUEL CELL SYSTEM COMPRISING SAME earlier filed in the KoreanIntellectual Property Office on 20 Mar. 2006 and there duly assignedSerial No. 10-2006-0025244.

BACKGROUND OF THE INVENTION

(a) Field of the Invention

The present invention relates to a membrane-electrode assembly for afuel cell, a method of manufacturing the same, and a fuel cell systemincluding the same.

(b) Description of the Related Art

A fuel cell is a power generation system for producing electrical energythrough an electrochemical redox reaction of an oxidant and hydrogen ina hydrocarbon-based material such as methanol, ethanol, or natural gas.

Such a fuel cell is a clean energy source that can replace fossil fuels.It includes a stack composed of unit cells, and produces various rangesof power output. Since it has four to ten times higher energy densitythan a small lithium battery, it has been high-lighted as a smallportable power source.

Representative exemplary fuel cells include a polymer electrolytemembrane fuel cell (PEMFC) and a direct oxidation fuel cell (DOFC). Thedirect oxidation fuel cell includes a direct methanol fuel cell, whichuses methanol as fuel.

The polymer electrolyte membrane fuel cell has an advantage of a highenergy density, but it also has problems in the need to carefully handlehydrogen gas and the requirement of accessory facilities such as a fuelreforming processor for reforming methane or methanol, natural gas, andthe like in order to produce hydrogen as the fuel gas.

On the contrary, a direct oxidation fuel cell has a lower energy densitythan that of the polymer electrolyte membrane fuel cell, but it hasadvantages of easy handling of fuel, being capable of operating at roomtemperature due to its low operation temperature, and no need foradditional fuel reforming processors.

In the above fuel cell, the stack that generates electricitysubstantially includes several to scores of unit cells stacked inmulti-layers, and each unit cell is formed of a membrane-electrodeassembly (MEA) and a separator (also referred to as a bipolar plate).The membrane-electrode assembly has an anode (also referred to as a fuelelectrode or an oxidation electrode) and a cathode (also referred to asan air electrode or a reduction electrode) attached to each other withan electrolyte membrane between them.

Fuel is supplied to an anode and absorbed in a catalyst thereof, and thefuel is oxidized to produce protons and electrons. The electrons aretransferred into a cathode via an external circuit, and the protons aretransferred into the cathode through a polymer electrolyte membrane. Anoxidant is supplied to the cathode, and the oxidant, protons, andelectrons are reacted on a catalyst at the cathode to produceelectricity along with water.

SUMMARY OF THE INVENTION

One embodiment of the present invention provides a membrane-electrodeassembly that has an improved interaction between a polymer electrolytemembrane and a catalyst layer and mass transfer properties due tomicropores.

Another embodiment of the present invention provides a method ofmanufacturing a membrane-electrode assembly.

Yet another embodiment of the present invention provides a fuel cellsystem that includes a membrane-electrode assembly.

According to one embodiment of the present invention, amembrane-electrode assembly for a fuel cell including a polymerelectrolyte membrane; and a first electrode and a second electrodeformed on both sides of the polymer electrolyte membrane and facing eachother, at least one of the first electrode and the second electrodecomprising: an electrode substrate; and a catalyst layer formed on theelectrode substrate, the catalyst layer comprising a first catalystlayer including a first catalyst grown from the polymer electrolytemembrane toward the electrode substrate and a second catalyst layerincluding a second catalyst covering the first catalyst layer.

The first catalyst is grown in a shape selected from the groupconsisting of a nano-nodule, a micro-nodule, a nanohorn, a nanorod, ananofiber, and combinations thereof. The first catalyst layer has anaverage height ranging from 50 nm to 5 mm from the polymer electrolytemembrane.

A thickness ratio of the first catalyst layer and the second catalystlayer ranges from 1/2500:1 to 1/25:1.

The first metal catalyst is selected from the group consisting of Pt,Ru, Au, W, Pd, Fe, and alloys thereof.

The second metal catalyst includes at least one selected from the groupconsisting of platinum, ruthenium, osmium, a platinum-ruthenium alloy, aplatinum-osmium alloy, a platinum-palladium alloy, a platinum-M alloy,and combinations thereof, where M is transition elements selected fromthe group consisting of Ga, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Sn, Mo,W, Rh, Ru, and combinations thereof.

The catalyst layer may further include a support which is formed on thepolymer electrolyte membrane and on which the first catalyst is grown,and the support is selected from the group consisting of Ce, Ni, Mn, Fe,Co, Zr, zirconia, polyoxometalate, heteropoly acid (HPA), andcombinations thereof.

According to another embodiment of the present invention, a method ofmanufacturing a membrane-electrode assembly includes providing a polymerelectrolyte membrane; forming a first electrode on one side of thepolymer electrolyte membrane and a second electrode on the other side ofthe polymer electrolyte membrane, said at least one of the firstelectrode and the second electrode being formed by: growing a firstcatalyst on one side of the polymer electrolyte membrane toward anelectrode substrate to form a first catalyst layer; and providing asecond catalyst layer including a second catalyst and an electrodesubstrate to form the second catalyst layer interposed between the firstcatalyst layer and the electrode substrate.

The providing of the second catalyst layer and the electrode substratemay include forming the second catalyst layer on the first catalystlayer and mounting the electrode substrate on the second catalyst layer.Alternatively, the providing of the second catalyst layer and theelectrode substrate may include forming the second catalyst layer on theelectrode substrate and mounting the second catalyst layer formed on theelectrode substrate on the first catalyst layer.

The first catalyst layer may be formed using a method selected from thegroup consisting of spraying, sputtering, physical vapor deposition(PVD), plasma enhanced chemical vapor deposition (PECVD), thermalchemical deposition, ion beam evaporation, vacuum thermal evaporation,laser ablation, thermal evaporation, electron beam evaporation, andcombinations thereof.

The second catalyst layer may be formed using a method selected from thegroup consisting of spraying, transferring, screen printing, andcombinations thereof.

According to yet another embodiment of the present invention, a fuelcell system including an electricity generating element includes theabove membrane-electrode assembly and a separator positioned at eachside of the membrane-electrode assembly, a fuel supplier that suppliesthe electricity generating element with fuel, and an oxidant supplierthat supplies the electricity generating element with an oxidant, isprovided.

BRIEF DESCRIPTION OF THE DRAWINGS

A more complete appreciation of the invention, and many of the attendantadvantages thereof, will be readily apparent as the same becomes betterunderstood by reference to the following detailed description whenconsidered in conjunction with the accompanying drawings in which likereference symbols indicate the same or similar components, wherein:

FIG. 1 is a schematic cross-sectional view showing a membrane-electrodeassembly according to an embodiment of the present invention;

FIG. 2 is a schematic diagram showing the structure of a fuel cellsystem according to another embodiment of the present invention; and

FIG. 3 is a cross-sectional view showing the first catalyst layer grownon a polymer electrolyte membrane in a nano-nodule shape in themembrane-electrode assembly according to Example 1.

DETAILED DESCRIPTION OF THE INVENTION

An exemplary embodiment of the present invention will hereinafter bedescribed in detail with reference to the accompanying drawings.

A membrane-electrode assembly of a fuel cell is composed of a polymerelectrolyte membrane, and an anode and a cathode arranged at each sideof the polymer electrolyte membrane. The membrane-electrode assemblygenerates electricity through oxidation of fuel and reduction of anoxidant. The reaction of generating electricity at a membrane-electrodeassembly actively occurs when the polymer electrolyte membrane has alarge interaction with a catalyst layer. That is to say, the polymerelectrolyte membrane should have excellent interface adhesion to anelectrode and also a large contact area at the interface therewith.

Therefore, according to one embodiment of the present invention, acatalyst layer includes a catalyst grown on a polymer electrolytemembrane toward an electrode substrate, and thereby increases theinteraction between the catalyst layer and the polymer electrolytemembrane. In addition, it includes micropores inside the catalyst layer,and thereby facilitates mass transfer toward the polymer electrolytemembrane, improving performance of a fuel cell.

In a membrane-electrode assembly according to one embodiment of thepresent invention, the catalyst layer includes a first catalyst layerincluding a first catalyst (e.g., a first metal catalyst) that growsfrom the polymer electrolyte membrane toward the electrode substrate anda second catalyst layer including a second catalyst (e.g., a secondmetal catalyst) covering the first catalyst layer.

FIG. 1 is a schematic cross-sectional view showing a membrane-electrodeassembly according to an embodiment of the present invention. As shownin FIG. 1, a catalyst layer with a double-layered structure is formed atboth sides of an anode and a cathode, but a membrane-electrode assemblyof the present invention is not limited thereto and a catalyst layerwith a double-layered structure can be formed on only one side of ananode and a cathode.

Referring to FIG. 1, a membrane-electrode assembly 10 of an embodimentof the present invention includes a polymer electrolyte membrane 20 andelectrodes 30 and 30′ for a fuel cell disposed at both sides of thepolymer electrolyte membrane 20. The electrodes 30 and 30′ includeelectrode substrates 70 and 70′ and catalyst layers 60 and 60′ formedthereon.

In the membrane-electrode assembly 10, an electrode 30 disposed at oneside of a polymer electrolyte membrane 20 is called an anode (or acathode), when the other electrode 30′ disposed at the other side of thepolymer electrolyte membrane 20 is called a cathode (or an anode). Fuelpasses an electrode substrate 70 and is transferred to a catalyst layer60 in the anode 30, and generates hydrogen ions and electrons by anoxidation reaction of the fuel. The polymer electrolyte membrane 20transfers the hydrogen ions generated from the anode 30 to the cathode30′. The catalyst layer 60′ catalyzes the reduction reaction of oxidantwhich passes an electrode substrate 70′ and transferred to a catalyst60′, producing water with the hydrogen ions supplied from the catalystlayer 60 through the polymer electrolyte membrane 20.

The catalyst layers 60 and 60′ catalyze oxidation of fuel and reductionof an oxidant, and include first catalyst layers 40 and 40′ and secondcatalyst layers 50 and 50′.

The first catalyst layers 40 and 40′ include a first metal catalystgrown toward electrode substrates 70 and 70′ on a polymer electrolytemembrane 20.

More specifically, the first catalyst layer is formed by growing a firstmetal catalyst toward an electrode substrate on a polymer electrolytemembrane in a deposition method, a chemical adsorption method, or thelike. In the above methods, a first metal catalyst directly grows in ashape such as nano-nodules, micro-nodules, nanohorns, nanorods, ornanofiber on a polymer electrolyte membrane by static electricity andinteraction among metal particles, and thereby increases adhesionbetween the polymer electrolyte membrane and a catalyst layer, leadingto facilitating mass transfer toward the polymer electrolyte membrane.

When the first metal catalyst grows in a shape such as nano-nodules,micro-nodules, nanohorns, nanorods, or nanofiber on a polymerelectrolyte membrane, it may be in a range of a nano-size to amicro-size, and in particular at an average height ranging from 50 nm to5 μm and preferably 200 nm to 2 μm. When the first metal catalyst has anaverage height of more than 5 μm, it may have a relatively small contactarea with the second metal catalyst, having little effect. On the otherhand, when it has an average height of less than 50 nm, it may have highdensity on the surface of a polymer electrolyte membrane so that thecatalyst layer is too dense and flat.

The location where the first metal catalyst grows or its spacing can beregulated by a pattern or roughness on the surface of a polymerelectrolyte membrane when it is disposed thereon. The first metalcatalyst may be formed with a spacing of 1 nm to 1 mm, and preferably 10to 500 nm, considering the case that the part of the second metalcatalyst is positioned among the grown first metal catalyst. When thefirst metal catalyst grows with a spacing of less than 1 nm, the secondmetal catalyst cannot enter among the grown first metal catalyst, or thegrown first metal catalyst may still have empty space among it,increasing membrane resistance. On the other hand, when of the spacingis more than 1 nm, the first metal catalyst may have a relatively smallcontact area with the second metal catalyst, having little catalysteffect.

In addition, the first metal catalyst can grow in a shape such asnano-nodules, micro-nodules, nanohorns, nanorods, or nanofiber, andthereby form micropores inside a catalyst layer. The micropores formedinside a catalyst layer can facilitate mass transfer toward a polymerelectrolyte membrane.

The first metal catalyst may be selected from the group consisting ofPt, Ru, Au, W, Pd, Fe, and alloys thereof. According to one embodiment,Pt, W, Au, or alloys thereof may be suitable for the first metalcatalyst. W and Au can advantageously provide a hydroxyl group, which isto be bound with Ru, and can thereby easily remove carbon monoxide boundwith Pt, improving catalyst effects.

When forming a first catalyst layer including the first metal catalyst,a support with high ion conductivity may be first attached to a polymerelectrolyte membrane to enhance an attachment rate of the first metalcatalyst to the polymer electrolyte membrane and to grow it in apredetermined shape. Accordingly, the first catalyst layer may include asupport with high ion conductivity as well as the first metal catalyst.The support may include at least one selected from the group consistingof a metal such as Ce, Ni, Mn, Fe, Co, Zr; zirconia; polyoxometalate(POM) such as polytungstate or polymolybdate; heteropoly acid (HPA), andcombinations thereof. According to one embodiment, Ce is suitable.

On the first catalyst layers 40 and 40′, the second catalyst layers 50and 50′ including a second metal catalyst are disposed to cover thefirst catalyst layer.

The second metal catalyst includes at least one selected from the groupconsisting of platinum, ruthenium, osmium, a platinum-ruthenium alloy, aplatinum-osmium alloy, a platinum-palladium alloy, a platinum-M alloy,or combinations thereof, where M is transition elements selected fromthe group consisting of Ga, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Sn, Mo,W, Rh, Ru, and combinations thereof. As described above, even though thesame catalyst may be used at an anode and a cathode, CO-tolerantplatinum-ruthenium alloy catalysts may be suitably used as an anodecatalyst in a direct oxidation fuel cell, since an anode catalyst may bepoisoned by CO. More specifically, non-limiting examples of theplatinum-based catalyst are selected from the group consisting of Pt,Pt/Ru, Pt/W, Pt/Ni, Pt/Sn, Pt/Mo, Pt/Pd, Pt/Fe, Pt/Cr, Pt/Co, Pt/Ru/W,Pt/Ru/Mo, Pt/Ru/V, Pt/Fe/Co, Pt/Ru/Rh/Ni, Pt/Ru/Sn/W, and combinationsthereof.

Such a metal catalyst may be used in a form of a metal itself (blackcatalyst), or one supported on a carrier. The carrier may include carbonsuch as graphite, denka black, ketjen black, acetylene black, carbonnanotubes, carbon nanofiber, carbon nanowire, carbon nanoballs, oractivated carbon, or an inorganic particulate such as alumina, silica,zirconia, or titania. The carbon can be generally used.

In a catalyst layer including first and second catalyst layers accordingto an embodiment of the present invention, a first metal catalyst may beincluded therein in a weight ratio ranging from such a small amount soas to be in an error range to a weight ratio of 98:2 between the firstand second metal catalysts. In terms of a thickness ratio, the catalystlayer may include the first and second catalyst layers in a thicknessratio ranging from 1/2500:1 to 1/25:1, and preferably from 1/650:1 to1/65:1. When the thickness ratio is within the given range, the firstand second catalysts may have increased contact areas, improvingcatalyst activity. However, when the thickness ratio is less than1/2500, the first metal catalyst may not grow well or not in apredetermined shape. On the other hand, when the thickness ratio is morethan 1/25, the first metal catalyst may be extremely dense and therebyjust cover the surface of a polymer electrolyte membrane rather thangrow in a predetermined shape.

Catalyst layers 60 and 60′ with the above structure can be included ineither one of an anode and a cathode. Since the first metal catalystplays a role of increasing the number of hydroxyl groups in thebifunctional mechanism, it is advantageously included in an anode.

The electrode substrates 70 and 70′ support the anode and the cathode,respectively, and provide a path for transferring fuel and an oxidant tothe catalyst layer 60 and 60′.

As for the electrode substrates 70 and 70′, a conductive substrate isused, for example carbon paper, carbon cloth, carbon felt, or a metalcloth (a porous film including a metal cloth fiber or a metallizedpolymer fiber), but is not limited thereto.

The electrode substrates 70 and 70′ may be treated with a fluorine-basedresin to be water-repellent to prevent deterioration of diffusionefficiency due to water generated during operation of a fuel cell. Thefluorine-based resin may include polyvinylidene fluoride,polytetrafluoroethylene, fluorinated ethylene propylene,polychlorotrifluoroethylene, a fluoroethylene polymer, or combinationsthereof.

A microporous layer (MPL, not shown) can be added between theaforementioned electrode substrates 70 and 70′ and catalyst layers 60and 60′ to increase reactant diffusion effects. The microporous layergenerally includes conductive powder materials, a binder, and an ionomerif required. The conductive power material may include, but is notlimited to, carbon powder, carbon black, acetylene black, activatedcarbon, carbon fiber, fullerene, nano-carbon, or combinations thereof.The nano-carbon may include a material such as carbon nanotubes, carbonnanofiber, carbon nanowire, carbon nanohorns, carbon nanorings, orcombinations thereof.

In the membrane-electrode assembly according to one embodiment of thepresent invention, a polymer electrolyte a membrane is disposed betweenthe anode and the cathode.

The polymer electrolyte membrane 20 plays a role of exchanging ions bytransferring protons produced at an anode catalyst layer 60 to a cathodecatalyst layer 60′. Therefore the polymer electrolyte membrane includesa proton conductive polymer that may be any polymer resin having acation exchange group selected from the group consisting of a sulfonicacid group, a carboxylic acid group, a phosphoric acid group, aphosphonic acid group, and derivatives thereof, at its side chain.Non-limiting examples of the polymer resin include at least one protonconductive polymer selected from the group consisting of fluoro-basedpolymers, benzimidazole-based polymers, polyimide-based polymers,polyetherimide-based polymers, polyphenylenesulfide-based polymerspolysulfone-based polymers, polyethersulfone-based polymers,polyetherketone-based polymers, polyether-etherketone-based polymers,and polyphenylquinoxaline-based polymers. In one embodiment, the protonconductive polymer is at least one selected from the group consisting ofpoly(perfluorosulfonic acid), poly(perfluorocarboxylic acid), acopolymer of tetrafluoroethylene and fluorovinylether having a sulfonicacid group, defluorinated polyetherketone sulfide, aryl ketone,poly(2,2′-(m-phenylene)-5,5′-bibenzimidazole), orpoly(2,5-benzimidazole).

In addition, when the perfluorosulfonic acid (trade name: NAFION) isused for the polymer electrolyte membrane, X in an ion exchange group(—SO₃X) at a terminal end of a side chain may be replaced with aunivalent ion such as sodium, potassium, and cesium, ortetrabutylammonium ion.

The substitution of an ion exchange group (—SO₃X) at the terminal end ofthe side chain in this way increases thermal stability of a fuel cell.Accordingly, even when a substrate is heat-treated at a high temperatureof over 200° C. during a hot-pressing process for fabricating amembrane-electrode assembly, a polymer resin therein cannot bedeteriorated, and thereby a lifespan of a fuel cell does not decrease.In addition, a polymer electrolyte membrane having an ion exchange groupsubstituted with an ion such as sodium, potassium, cesium,tetrabutylammonium, or the like, for example a sodium-type polymerelectrolyte membrane, is re-sulfonized when a catalyst layer is treatedwith a sulfonic acid later and thereby changes into a proton-typepolymer electrolyte membrane.

In addition, the polymer electrolyte membrane may increase a contactarea with a catalyst layer of an electrode, and thereby increase power.Further, it can have a surface roughness on one side or preferably bothsides to facilitate a growth of metal catalyst in a predetermined shape.

A manufacturing of the membrane-electrode assembly with the abovestructure is shown hereinbelow with an exemplary method.

The method of manufacturing a membrane-electrode assembly includesproviding a polymer electrolyte membrane; growing a first metal catalyston a polymer electrolyte membrane toward an electrode substrate to forma first catalyst layer; forming a second metal catalyst layer coveringthe first catalyst layer; and mounting an electrode substrate on thesecond catalyst layer. Alternatively, a method of manufacturing amembrane-electrode assembly includes providing a polymer electrolytemembrane; growing a first metal catalyst on a polymer electrolytemembrane toward an electrode substrate to form a first catalyst layer;forming a second catalyst layer on a electrode substrate; and mountingthe second catalyst layer on the electrode substrate on the firstcatalyst layer on the polymer electrolyte membrane.

More particularly, a polymer electrolyte membrane is prepared from acation exchange resin with hydrogen ion conductivity.

The polymer electrolyte membrane has no particular limit to the method,but can be fabricated in a common method by using a cation exchangeresin with hydrogen ion conductivity. The cation exchange resin withhydrogen ion conductivity may include the same as described above.

The polymer electrolyte membrane may have surface roughness that isformed by a plasma treatment on either or both surface thereof, etchingtreatment by using an acid solution, anodizing, corona treatment,rubbing, compression by using a plastic substrate with a predeterminedpattern, sand papering, sand blasting, and combinations thereof.

In addition, before growing a first metal catalyst on the polymerelectrolyte membrane, a support with high ion conductivity can beoptionally attached thereto to increase an attachment rate of the firstmetal catalyst.

Herein, the support is the same as described above. The support may beattached using a method selected from the group consisting ofsputtering, physical vapor deposition (PVD), plasma enhanced chemicalvapor deposition (PECVD), thermal chemical deposition, ion beamevaporation, vacuum thermal evaporation, laser ablation, thermalevaporation, electron beam evaporation, and combinations thereof.According to one embodiment, sputtering may be suitable.

The support may be attached at a temperature ranging from 0 to 25° C.This temperature range can increase an attachment rate of a support andfacilitate a first metal catalyst to easily grow in a predeterminedshape.

Next, a first catalyst layer is formed by growing a first metal catalyston a support attached to a polymer electrolyte membrane in a directionof an electrode substrate.

The first metal catalyst is the same as described above. The firstcatalyst layer may be formed using a method selected from the groupconsisting of spraying, sputtering, physical vapor deposition (PVD),plasma enhanced chemical vapor deposition (PECVD), thermal chemicaldeposition, ion beam evaporation, vacuum thermal evaporation, laserablation, thermal evaporation, electron beam evaporation, andcombinations thereof.

The above process may be performed at a temperature ranging from 0 to25° C. When performed in the above range, a first metal catalyst caneasily grow in a predetermined shape.

Therefore, the first metal catalyst may grow on a polymer electrolytemembrane or a support attached thereto in a direction of an electrode ina shape such as a nano-nodule, a micro-nodule, a nanohorn, a nanorod, ananofiber, or the like.

Then, after a second catalyst layer including a second metal catalyst isformed to cover the first catalyst layer, an electrode substrate isunited therewith, or after a second catalyst layer including a secondmetal catalyst is formed on an electrode substrate, they are united witha polymer electrolyte membrane including a first catalyst layer to forma membrane-electrode assembly.

The second metal catalyst is the same as described above. The secondcatalyst layer may be formed using a method selected from the groupconsisting of spraying, transferring, screen printing, and combinationsthereof.

The electrode substrate is the same as described above. A method ofuniting an electrode substrate with a polymer electrolyte membrane iswell-known in this related field so it is not necessary to describe itin detail.

The membrane-electrode assembly may include a catalyst shaped as anano-nodule, a micro-nodule, a nanohorn, a nanorod, or a nanofiber, andalso a catalyst layer including micropores. Accordingly, the polymerelectrolyte membrane in the membrane-electrode assembly may haveincreased interaction with the catalyst layer therein. In addition, masstransfer can be easily made through the polymer electrolyte membrane,improving cell characteristics.

According to an embodiment of the present invention, a fuel cell systemincluding the above membrane-electrode assembly is provided.

The fuel cell system includes at least one electricity generatingelement including the membrane-electrode assembly and a separator, afuel supplier for supplying the electricity generating element withfuel, and an oxidant supplier for supplying the electricity generatingelement with an oxidant.

The electricity generating element includes a membrane-electrodeassembly and a separator (also referred to as a bipolar plate), andplays a role of generating electricity through oxidation of fuel andreduction of an oxidant.

The fuel supplier plays a role of supplying the electricity generatingelement with fuel including hydrogen, and the oxidant supplier plays arole of supplying the electricity generating element with an oxidant.The oxidant includes oxygen or air. The fuel includes liquid or gaseoushydrogen, or a hydrocarbon-based fuel such as methanol, ethanol,propanol, butanol, or natural gas.

FIG. 2 shows a schematic structure of a fuel cell system 100 that willbe described in detail with reference to this accompanying drawing, asfollows. FIG. 2 illustrates a fuel cell system wherein fuel and anoxidant are provided to the electricity generating element 130 throughpumps 124 and 132, but the present invention is not limited to suchstructures. The fuel cell system of the present invention alternativelyincludes a structure wherein fuel and an oxidant are provided in adiffusion manner.

A fuel cell system 100 includes at least one electricity generatingelement 115 that generates electrical energy through an electrochemicalreaction of fuel and an oxidant, a fuel supplier 120 for supplying fuelto the electricity generating element 115, and an oxidant supplier 130for supplying an oxidant to the electricity generating element 115.

In addition, the fuel supplier 120 is equipped with a tank 122 thatstores fuel, and a pump 124 that is connected therewith. The fuel pump124 supplies fuel stored in the tank 122 with a predetermined pumpingpower.

The oxidant supplier 130, which supplies the electricity generatingelement 115 with an oxidant, is equipped with at least one pump 132 forsupplying an oxidant with a predetermined pumping power.

The electricity generating element 115 includes a membrane-electrodeassembly 112 that oxidizes hydrogen or fuel and reduces an oxidant, andseparators 114 and 114′ that are respectively positioned at oppositesides of the membrane-electrode assembly and supply hydrogen or fuel,and an oxidant, respectively. At least one electricity generatingelement 115 constitutes a stack 110.

The following examples illustrate the present invention in more detail.However, it is understood that the present invention is not limited bythese examples.

EXAMPLE 1

125 μm of a commercially available NAFION 115 membrane was respectivelytreated with 3% hydrogen peroxide at 100° C. and 0.5M of a NaOH aqueoussolution for one hour, and was thereafter washed with deionized water at100° C. for one hour to prepare a sodium-type polymer electrolytemembrane.

Then, Ce was sputtered at 20° C. and attached to one side of the polymerelectrolyte membrane. Then, Au as a first metal catalyst was grown onthe Ce through ion sputtering. Next, a slurry including a Pt—Ru alloy asa second metal catalyst was prepared, and was thereafter coated on thefirst catalyst layer with a spraying method to form a second metalcatalyst layer. The other side of the polymer electrolyte membrane wastreated the same way to form first and second catalyst layers.Accordingly, the polymer electrolyte membrane had cathode and anodecatalyst layers at respective sides.

Then, a commercially-available electrode substrate (SGL Carbon 31BC) wasphysically adhered to both sides of the polymer electrolyte membranehaving catalyst layers. The prepared membrane-electrode assembly wasinterposed between two sheets of gaskets and then between two separatorswith a predetermined-shaped gas channel and cooling channel, and wasfinally compressed together between copper end plates, fabricating asingle cell.

EXAMPLE 2

A single cell was fabricated by the same method as in Example 1, exceptthat the catalyst layer was formed only on an anode side of a polymerelectrolyte membrane.

EXAMPLE 3

A single cell was fabricated by the same method as in Example 1, exceptthat W was used as the first metal catalyst, and Pt/Ru supported ongraphite was used as the second metal catalyst, and in addition, acatalyst layer including the first and second metal catalysts was formedonly on the anode.

COMPARATIVE EXAMPLE 1

125 μm of a commercially-available NAFION 115 membrane was respectivelytreated with 3% hydrogen peroxide at 100° C. and 0.5M of a NaOH aqueoussolution for one hour, and was then washed with deionized water at 100°C. for one hour to prepare a sodium-type NAFION 115 membrane.

4.5 g of 10 wt % NAFION (NAFION®, Dupont Co.) water-based dispersionsolution was added to 3.0 g of Pt black (Hispec®1000, Johnson MattheyCo.) and Pt/Ru black (Hispec® 6000, Johnson Matthey Co.) catalysts in adrop-wise fashion. The mixture was mechanically agitated to prepare acomposition for forming a catalyst layer.

The composition for forming a catalyst layer was directly coated on oneside of the polymer electrolyte membrane through screen printing. Thecatalyst layer was formed with an area of 5×5 cm² and respectivelyloaded at 3 mg/cm². The other side of the polymer electrolyte membranewas treated the same way to respectively form cathode and anode catalystlayers on respective sides thereof.

Then, a commercially-available electrode substrate (SGL Carbon 31BC) wasphysically adhered to both sides of the polymer electrolyte membranehaving the catalyst layers. The membrane-electrode assembly wasinterposed between two sheets of gaskets and then between two separatorswith a gas channel and a cooling channel, and was finally compressedtogether between copper end plates, fabricating a single cell.

After the first metal catalyst was grown on a polymer electrolytemembrane in Example 1, it was examined regarding its cross-sectionthrough a scanning electron microscope. The results are provided in FIG.3.

As shown in FIG. 3, a first metal catalyst, Au, was identified to growon a polymer electrolyte membrane in a nano-nodule shape in an ionsputtering method.

As for a single cell according to Examples 1 and 2 and ComparativeExample 1, methanol was inserted into a cathode catalyst layer and airinto an anode catalyst layer, and then their output change was measured,depending on temperature of the cell and methanol concentration. Theresults are provided in the following Table 1.

TABLE 1 Power density at 60° C. (mW/cm² at 0.4 V) Example 1 100 Example2 120 Comparative Example 1 80

As shown in Table 1, a single cell of Examples 1 and 2, which includes anano-nodule metal catalyst, turned out to have much better outputdensity than that of Comparative Example 1. The single cell of Example 1includes a nano-nodule catalyst grown on a polymer electrolyte membrane,and could thereby have increased catalyst activity. In addition, sincemicropores were formed inside a catalyst layer, a material and an ioncan easily transfer toward the polymer electrolyte membrane.

Therefore, a membrane-electrode assembly of the embodiments of thepresent invention can have increased interactions between a polymerelectrolyte membrane and a catalyst, and also micropores inside acatalyst layer, and thereby make mass easily transferred toward thepolymer electrolyte membrane. Accordingly, a fuel cell including themembrane-electrode assembly of the present invention can have excellentfuel cell performance.

While this invention has been described in connection with what ispresently considered to be practical exemplary embodiments, it is to beunderstood that the invention is not limited to the disclosedembodiments, but, on the contrary, is intended to cover variousmodifications and equivalent arrangements included within the spirit andscope of the appended claims.

1. A membrane-electrode assembly for a fuel cell, comprising: a polymerelectrolyte membrane; and a first electrode and a second electrodeformed on both sides of the polymer electrolyte membrane and facing eachother, at least one of the first electrode and the second electrodecomprising: an electrode substrate; and a catalyst layer comprising afirst metal catalyst layer consisting of a metal selected from the groupconsisting of Pt, Ru, Au, W, Pd, Fe, and alloys thereof and directlygrown on the polymer electrolyte membrane toward the electrode substratewith a shape of a nano-nodule; and a second catalyst layer including asecond metal catalyst covering the first metal catalyst layer.
 2. Themembrane-electrode assembly of claim 1, wherein the first metal catalystlayer has an average height ranging from 50 nm to 5 μm measured from thepolymer electrolyte membrane.
 3. The membrane-electrode assembly ofclaim 1, wherein the weight of the first metal catalyst is not greaterthan 98 percent by weight based on the total weight of the first metalcatalyst and the second metal catalyst.
 4. The membrane-electrodeassembly of claim 1, wherein a thickness ratio of the first metalcatalyst layer and the second catalyst layer ranges from 1/2500:1 to1/25:1.
 5. The membrane-electrode assembly of claim 1, wherein thecatalyst layer has a micropore.
 6. The membrane-electrode assembly ofclaim 1, wherein the second metal catalyst comprises at least oneselected from the group consisting of platinum, ruthenium, osmium, aplatinum-ruthenium alloy, a platinum-osmium alloy, a platinum-palladiumalloy, a platinum-M alloy, and combinations thereof, where M istransition elements selected from the group consisting of Ga, Ti, V, Cr,Mn, Fe, Co, Ni, Cu, Zn, Sn, Mo, W, Rh, Ru, and combinations thereof. 7.The membrane-electrode assembly of claim 1, wherein the catalyst layerfurther comprises a support which is formed on the polymer electrolytemembrane and on which the first metal catalyst layer is grown, and thesupport is selected from the group consisting of Ce, Ni, Mn, Fe, Co, Zr,zirconia, polyoxometalate, heteropoly acid (HPA), and combinationsthereof.
 8. The membrane-electrode assembly of claim 1, wherein eitherthe first electrode or the second electrode is an anode.
 9. Themembrane-electrode assembly of claim 1, wherein at least one of thefirst electrode and the second electrode comprises a microporous layerformed between the electrode substrate and the catalyst layer.
 10. Afuel cell system comprising: an electricity generating elementcomprising: a membrane-electrode assembly comprising: a polymerelectrolyte membrane; and an anode and a cathode formed on both sides ofthe polymer electrolyte membrane and facing each other, at least one ofthe anode and the cathode comprising: an electrode substrate; and acatalyst layer comprising a first metal catalyst layer consisting of ametal selected from the group consisting of Pt, Ru, Au, W, Pd, Fe, andalloys thereof and directly grown on the polymer electrolyte membranetoward the electrode substrate with a shape of a nano-nodule; and asecond catalyst layer including a second metal catalyst covering thefirst metal catalyst layer; and a separator positioned at each side ofthe membrane-electrode assembly; a fuel supplier supplying theelectricity generating element with fuel; and an oxidant suppliersupplying the electricity generating element with an oxidant.
 11. Thefuel cell system of claim 10, wherein the catalyst layer has amicropore.
 12. The fuel cell system of claim 10, wherein the first metalcatalyst layer has an average height ranging measured from 50 nm to 5 μmmeasured from the polymer electrolyte membrane, and a thickness ratio ofthe first metal catalyst layer and the second catalyst layer ranges from1/2500:1 to 1/25:1.
 13. The fuel cell system of claim 10, wherein thesecond metal catalyst comprises at least one selected from the groupconsisting of platinum, ruthenium, osmium, a platinum-ruthenium alloy, aplatinum-osmium alloy, a platinum-palladium alloy, a platinum-M alloy,and combinations thereof, where M is transition elements selected fromthe group consisting of Ga, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Sn, Mo,W, Rh, Ru, and combinations thereof.
 14. The fuel cell system of claim10, wherein the catalyst layer further comprises a support which isformed on the polymer electrolyte membrane and on which the first metalcatalyst layer is grown, and the support is selected from the groupconsisting of Ce, Ni, Mn, Fe, Co, Zr, zirconia, polyoxometalate,heteropoly acid (HPA), and combinations thereof.
 15. The fuel cellsystem of claim 10, wherein the catalyst layer is an anode catalystlayer.
 16. A membrane-electrode assembly for a fuel cell, comprising: apolymer electrolyte membrane; and a first electrode and a secondelectrode formed on both sides of the polymer electrolyte membrane andfacing each other, at least one of the first electrode and the secondelectrode comprising: an electrode substrate; and a catalyst layercomprising a first catalyst layer including a first metal catalyst layerdirectly grown on the polymer electrolyte membrane toward the electrodesubstrate with a shape selected from the group consisting of a nanohorn,a nanorod, a nanofiber, and combinations thereof and a second catalystlayer including a second catalyst covering the first catalyst layer. 17.A fuel cell system comprising: an electricity generating elementcomprising: a membrane-electrode assembly comprising: a polymerelectrolyte membrane; and an anode and a cathode formed on both sides ofthe polymer electrolyte membrane and facing each other, at least one ofthe anode and the cathode comprising: an electrode substrate; and acatalyst layer comprising a first catalyst layer including a first metalcatalyst layer directly grown on the polymer electrolyte membrane towardthe electrode substrate with a shape selected from the group consistingof a nanohorn, a nanorod, a nanofiber, and combinations thereof and asecond catalyst layer including a second catalyst covering the firstcatalyst layer; and a separator positioned at each side of themembrane-electrode assembly; a fuel supplier supplying the electricitygenerating element with fuel; and an oxidant supplier supplying theelectricity generating element with an oxidant.